Crystalline chromium deposit

ABSTRACT

An electroplating bath and a process for electrodepositing a crystalline chromium deposit on a substrate, in which the electroplating bath comprising trivalent chromium and a source of divalent sulfur, and substantially free of hexavalent chromium; immersing a substrate in the electroplating bath; and applying an electrical current to deposit a crystalline chromium deposit on the substrate, wherein the chromium deposit is crystalline as deposited, and/or has a lattice parameter of 2.8895+/−0.0025 Å, and/or the crystalline chromium deposit has a {111} preferred orientation.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a division of and claims priority under 35U.S.C. §120 to U.S. application Ser. No. 11/692,523, filed 28 Mar. 2007,now U.S. Pat. No. 7,887,930, which is related to and claims benefitunder 35 U.S.C. §119 to U.S. Provisional Application No. 60/788,387,filed 31 Mar. 2006, the entirety of both of which is hereby incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates generally to electrodeposited crystallinechromium deposited from trivalent chromium baths, methods forelectrodepositing such chromium deposits and articles having suchchromium deposits applied thereto.

BACKGROUND

Chromium electroplating began in the early twentieth or late 19^(th)century and provides a superior functional surface coating with respectto both wear and corrosion resistance. However, in the past, thissuperior coating, as a functional coating (as opposed to a decorativecoating), has only been obtained from hexavalent chromium electroplatingbaths. Chromium electrodeposited from hexavalent chromium baths isdeposited in a crystalline form, which is highly desirable. Amorphousforms of chromium plate are not useful. The chemistry that is used inpresent technology is based on hexavalent chromium ions, which areconsidered carcinogenic and toxic. Hexavalent chromium platingoperations are subject to strict and severe environmental limitations.While industry has developed many methods of working with hexavalentchromium to reduce the hazards, both industry and academia have for manyyears searched for a suitable alternative.

Given the importance and superiority of chromium plating, the mostobvious alternative source of chromium for chromium plating is trivalentchromium. Trivalent chromium salts are much less hazardous to health andthe environment than hexavalent chromium compounds. Many differenttrivalent chromium electrodeposition baths have been tried and testedover the years. However, none of such trivalent chromium baths havesucceeded in producing a reliably consistent chromium deposit that iscomparable to that obtained from hexavalent chromium electrodepositionprocesses.

Hexavalent chromium is very toxic and is subject to regulatory controlsthat trivalent chromium is not. The most recent OSHA rule for hexavalentchromium exposure was published in 29 CFR Parts 1910, 1915, et al.,Occupational Exposure to Hexavalent Chromium; Final Rule. In this Rule,substitution is described as an “ideal (engineering) control measure”and “replacement of a toxic materials with a less hazardous alternativeshould always be considered” (Federal Register/Vol. 71, No. 39/Tuesday,Feb. 28, 2006/Rules and Regulations pp. 10345). Thus, there are stronggovernment-based mandates to replace hexavalent chromium with anotherform of chromium. However, until the present invention, no process hasbeen successful in electrodepositing a reliably consistent crystallinechromium deposit from a trivalent or other non-hexavalent chromiumelectroplating bath.

In general, in the prior art, all of the trivalent chromiumelectrodeposition processes form an amorphous chromium deposit. While itis possible to anneal the amorphous chromium deposit at about 350 to370° C., and create thereby a crystalline chromium deposit, theannealing results in the formation of macrocracks, which areundesirable, rendering the chromium deposit essentially useless.Macrocracks are defined as cracks that extend through the entirethickness of the chromium layer, down to the substrate. Since themacrocracks reach the substrate, thus giving ambient materials access tothe substrate, the chromium deposit cannot provide its function ofcorrosion resistance. The macrocracks are believed to arise from theprocess of crystallization, since the desired body-centered cubiccrystalline form has a smaller volume than does the as-depositedamorphous chromium deposit and the resulting stress causes the chromiumdeposit to crack, forming the macrocracks. By contrast, crystallinechromium deposits from hexavalent electrodeposition processes generallyinclude microcracks that are smaller and extend only a fraction of thedistance from the surface of the deposit towards the substrate, and donot extend through the entire thickness of the chromium deposit. Thereare some instances in which a crack-free chromium deposit from ahexavalent chromium electrolyte can be obtained. The frequency ofmicrocracks in chromium from hexavalent chromium electrolytes, wherepresent, is on the order of 40 or more cracks per centimeter, while thenumber of macrocracks in amorphous deposits from trivalent chromiumelectrolytes annealed to form crystalline chromium, where present, isabout an order of magnitude less. Even with the much lower frequency,the macrocracks render the trivalent chromium derived crystallinedeposit unacceptable for functional use. Functional chromium depositsneed to provide both wear resistance and corrosion resistance, and thepresence of macrocracks renders the article subject to corrosion, andthus such chromium deposits are unacceptable.

Trivalent chromium electrodeposition processes can successfully deposita decorative chromium deposit. However, decorative chromium is notfunctional chromium, and is not capable of providing the benefits offunctional chromium.

While it would appear to be a simple matter to apply and adapt thedecorative chromium deposit to functional chromium deposits, this hasnot occurred. Rather, for years the goal has continued to elude the manyefforts directed at solving this problem and reaching the goal of atrivalent chromium electrodeposition process that can form a crystallinechromium deposit.

Another reason for seeking a trivalent chromium electrodepositionprocess is that trivalent chromium based processes theoretically requireabout half as much electrical energy as a hexavalent process. UsingFaraday's law, and assuming the density of chromium to be 7.14 g/cm³ theplating rate of a 25% cathodic efficiency process with 50 A/dm² appliedcurrent density is 56.6 microns per dm² per hour for a hexavalentchromium plating process. With similar cathodic efficiencies and currentdensity a deposit of chromium from the trivalent state would have twicethe thickness in the same time period.

For all these reasons, a long-felt need remains for a functionalcrystalline-as-deposited chromium deposit, an electrodeposition bath andprocess capable of forming such a chromium deposit and articles madewith such a chromium deposit, in which the chromium deposit is free ofmacrocracks and is capable of providing functional wear and corrosionresistance characteristics comparable to the functional hard chromiumdeposit obtained from a hexavalent chromium electrodeposition process.The urgent need for a bath and process capable of providing acrystalline functional chromium deposit from a bath substantially freeof hexavalent chromium heretofore has not been satisfied.

SUMMARY

The present invention provides a chromium deposit which is crystallinewhen deposited, and which is deposited from a trivalent chromiumsolution.

The present invention, although possibly useful for formation ofdecorative chromium deposits, is primarily directed to functionalchromium deposits, and in particular for functional crystalline chromiumdeposits which heretofore have only been available through hexavalentchromium electrodeposition processes.

The present invention provides a solution to the problem of providing acrystalline functional chromium deposit from a trivalent chromium bathsubstantially free of hexavalent chromium, but which nevertheless iscapable of providing a product with functional characteristicssubstantially equivalent to those obtained from hexavalent chromiumelectrodeposits. The invention provides a solution to the problem ofreplacing hexavalent chromium plating baths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes three X-ray diffraction patterns (Cu k alpha) ofcrystalline chromium deposited in accordance with an embodiment of thepresent invention and with hexavalent chromium of the prior art.

FIG. 2 is a typical X-ray diffraction pattern (Cu k alpha) of amorphouschromium from a trivalent chromium bath of the prior art.

FIG. 3 is a typical X-ray diffraction pattern (Cu k alpha) showing theprogressive effect of annealing an amorphous chromium deposit from atrivalent chromium bath of the prior art.

FIG. 4 is a series of electron photomicrographs showing themacrocracking effect of annealing an initially amorphous chromiumdeposit from a trivalent chromium bath of the prior art.

FIG. 5 is a typical X-ray diffraction pattern (Cu k alpha) of acrystalline as-deposited chromium deposit in accordance with anembodiment of the present invention.

FIG. 6 is a series of typical X-ray diffraction patterns (Cu k alpha) ofcrystalline chromium deposits in accordance with embodiments of thepresent invention.

FIG. 7 is a graphical chart illustrating how the concentration of sulfurin one embodiment of a chromium deposit relates to the crystallinity ofthe chromium deposit.

FIG. 8 is a graphical chart comparing the crystal lattice parameter, inAngstroms (Å) for (1) a crystalline chromium deposit in accordance withan embodiment of the present invention, compared with (2) crystallinechromium deposits from hexavalent chromium baths and (3) annealedamorphous-as-deposited chromium deposits.

FIG. 9 is a typical X-ray diffraction pattern (Cu k alpha) showing theprogressive effect of increasing amounts of thiosalicylic acid showingthe reliably consistent (222) reflection, {111} preferred orientation,crystalline chromium deposit from a trivalent chromium bath inaccordance with an embodiment of the present invention.

It should be appreciated that the process steps and structures describedbelow do not form a complete process flow for manufacturing partscontaining the functional crystalline chromium deposit of the presentinvention. The present invention can be practiced in conjunction withfabrication techniques currently used in the art, and only so much ofthe commonly practiced process steps are included as are necessary foran understanding of the present invention.

DETAILED DESCRIPTION

As used herein, a decorative chromium deposit is a chromium deposit witha thickness less than one micron, and often less than 0.8 micron,typically applied over an electrodeposited nickel or nickel alloycoating, or over a series of copper and nickel or nickel alloy coatingswhose combined thicknesses are in excess of three microns.

As used herein, a functional chromium deposit is a chromium depositapplied to (often directly to) a substrate such as strip steel ECCS(Electrolytically Chromium Coated Steel) where the chromium thickness isgenerally greater than 0.8 or 1 micron, and is used for industrial, notdecorative, applications. Functional chromium deposits are generallyapplied directly to a substrate. Industrial coatings take advantage ofthe special properties of chromium, including its hardness, itsresistance to heat, wear, corrosion and erosion, and its low coefficientof friction. Even though it has nothing to do with performance, manyusers want the functional chromium deposits to be decorative inappearance. The thickness of the functional chromium deposit may rangefrom the above-noted 0.8 or 1 micron to 3 microns or much more. In somecases, the functional chromium deposit is applied over a ‘strike plate’such as nickel or iron plating on the substrate or a ‘duplex’ system inwhich the nickel, iron or alloy coating has a thickness greater thanthree microns and the chromium thickness generally is in excess of threemicrons. Functional chromium plating and deposits are often referred toas “hard” chromium plating and deposits.

Decorative chromium plating baths are concerned with thin chromiumdeposits over a wide plating range so that articles of irregular shapeare completely covered. Functional chromium plating, on the other hand,is designed for thicker deposits on regularly shaped articles, whereplating at a higher current efficiency and at higher current densitiesis important. Previous chromium plating processes employing trivalentchromium ion have generally been suitable for forming only “decorative”finishes. The present invention provides “hard” or functional chromiumdeposits, but is not so limited, and can be used for decorative chromiumfinishes. “Hard” or “functional” and “decorative” chromium deposits areknown terms of art.

As used herein, when used with reference to, e.g., an electroplatingbath or other composition, “substantially free of hexavalent chromium”means that the electroplating bath or other composition so described isfree of any intentionally added hexavalent chromium. As will beunderstood, such a bath or other composition may contain trace amountsof hexavalent chromium present as an impurity in materials added to thebath or composition or as a by-product of electrolytic or chemicalprocesses carried out with bath or composition.

As used herein, the term “preferred orientation” carries the meaningthat would be understood by those of skill in the crystallographic arts.Thus, “preferred orientation” is a condition of polycrystallineaggregate in which the crystal orientations are not random, but ratherexhibit a tendency for alignment with a specific direction in the bulkmaterial. Thus, a preferred orientation may be, for example, {100},{110}, {111} and integral multiples thereof, such as (222).

The present invention provides a reliably consistent body centered cubic(BCC) crystalline chromium deposit from a trivalent chromium bath, whichbath is substantially free of hexavalent chromium, and in which thechromium deposit is crystalline as deposited, without requiring furthertreatment to crystallize the chromium deposit. Thus, the presentinvention provides a solution to the long-standing, previously unsolvedproblem of obtaining a reliably consistent crystalline chromium depositfrom an electroplating bath and a process which are substantially freeof hexavalent chromium.

In one embodiment, the crystalline chromium deposit of the presentinvention is substantially free of macrocracks, using standard testmethods. That is, in this embodiment, under standard test methods,substantially no macrocracks are observed when samples of the chromiumdeposited are examined.

In one embodiment, the crystalline chromium deposit in accordance withthe present invention has a cubic lattice parameter of 2.8895+/−0.0025Angstroms (Å). It is noted that the term “lattice parameter” is alsosometimes used as “lattice constant”. For purposes of the presentinvention, these terms are considered synonymous. It is noted that forbody centered cubic crystalline chromium, there is a single latticeparameter, since the unit cell is cubic. This lattice parameter is moreproperly referred to as a cubic lattice parameter, but herein isreferred to simply as the “lattice parameter”. In one embodiment, thecrystalline chromium deposit in accordance with the present inventionhas a lattice parameter of 2.8895 Å+/−0.0020 Å. In another embodiment,the crystalline chromium deposit in accordance with the presentinvention has a lattice parameter of 2.8895 Å+/−0.0015 Å. In yet anotherembodiment, the crystalline chromium deposit in accordance with thepresent invention has a lattice parameter of 2.8895 Å+/−0.0010 Å. Somespecific examples are provided herein of crystalline chromium depositshaving lattice parameters within these ranges.

Pyrometallurgical, elemental crystalline chromium has a latticeparameter of 2.8839 Å.

Crystalline chromium electrodeposited from a hexavalent chromium bathhas a lattice parameter ranging from about 2.8809 Å to about 2.8858 Å.

Annealed electrodeposited trivalent amorphous-as-deposited chromium hasa lattice parameter ranging from about 2.8818 Å to about 2.8852 Å, butalso has macrocracks.

Thus, the lattice parameter of the chromium deposit in accordance withthe present invention is larger than the lattice parameter of otherknown forms of crystalline chromium. Although not to be bound by theory,it is considered that this difference may be due to the incorporation ofheteroatoms, such as sulfur, nitrogen, carbon, oxygen and/or hydrogen inthe crystal lattice of the crystalline chromium deposit obtained inaccordance with the present invention.

In one embodiment, the crystalline chromium deposit in accordance withthe invention has a {111} preferred orientation.

In one embodiment, the crystalline chromium deposit is substantiallyfree of macrocracking. In one embodiment, the crystalline chromiumdeposit does not form macrocracks when heated to a temperature up toabout 300° C. In one embodiment, the crystalline chromium deposit doesnot change its crystalline structure when heated to a temperature up toabout 300° C.

In one embodiment, the crystalline chromium deposit further includescarbon, nitrogen and sulfur in the chromium deposit.

In one embodiment, the crystalline chromium deposit contains from about1.0 wt. % to about 10 wt. % sulfur. In another embodiment, the chromiumdeposit contains from about 1.5 wt. % to about 6 wt. % sulfur. Inanother embodiment, the chromium deposit contains from about 1.7 wt. %to about 4 wt. % sulfur. The sulfur is in the deposit present aselemental sulfur and may be a part of crystal lattice, i.e., replacingand thus taking the position of a chromium atom in the crystal latticeor taking a place in the tetrahedral or octahedral hole positions anddistorting the lattice. In one embodiment, the source of sulfur may be adivalent sulfur compound. More details on exemplary sulfur sources areprovided below. In one embodiment, instead of or in addition to sulfur,the deposit contains selenium and/or tellurium.

It is noted that some forms of crystalline chromium deposited fromhexavalent chromium baths contain sulfur, but the sulfur content of suchchromium deposits is substantially lower than the sulfur content of thecrystalline chromium deposits in accordance with the present invention.

In one embodiment, the crystalline chromium deposit contains from about0.1 to about 5 wt % nitrogen. In another embodiment, the crystallinechromium deposit contains from about 0.5 to about 3 wt % nitrogen. Inanother embodiment the crystalline chromium deposit contains about 0.4weight percent nitrogen.

In one embodiment, the crystalline chromium deposit contains from about0.1 to about 5 wt % carbon. In another embodiment, the crystallinechromium deposit contains from about 0.5 to about 3 wt % carbon. Inanother embodiment the crystalline chromium deposit contains about 1.4wt. % carbon. In one embodiment, the crystalline chromium depositcontains an amount of carbon less than that amount which renders thechromium deposit amorphous. That is, above a certain level, in oneembodiment, above about 10 wt. %, the carbon renders the chromiumdeposit amorphous, and therefore takes it out of the scope of thepresent invention. Thus, the carbon content should be controlled so thatit does not render the chromium deposit amorphous. The carbon may bepresent as elemental carbon or as carbide carbon. If the carbon ispresent as elemental, it may be present either as graphitic or asamorphous.

In one embodiment, the crystalline chromium deposit contains from about1.7 wt. % to about 4 wt. % sulfur, from about 0.1 wt. % to about 5 wt. %nitrogen, and from about 0.1 wt. % to about 10 wt. % carbon.

The crystalline chromium deposit of the present invention iselectrodeposited from a trivalent chromium electroplating bath. Thetrivalent chromium bath is substantially free of hexavalent chromium. Inone embodiment, the bath is free of detectable amounts of hexavalentchromium. The trivalent chromium may be supplied as chromic chloride,CrCl₃, chromic fluoride, CrF₃, chromic nitrate, Cr(NO₃)₃, chromic oxideCr₂O₃, chromic phosphate CrPO₄, or in a commercially available solutionsuch as chromium hydroxy dichloride solution, chromic chloride solution,or chromium sulfate solution, e.g., from McGean Chemical Company orSentury Reagents. Trivalent chromium is also available as chromiumsulfate/sodium or potassium sulfate salts, e.g., Cr(OH)SO₄.Na₂SO₄, oftenreferred to as chrometans or kromsans, chemicals often used for tanningof leather, and available from companies such as Elementis, LancashireChemical, and Soda Sanayii. As noted below, the trivalent chromium mayalso be provided as chromic formate, Cr(HCOO)₃ from Sentury Reagents.

The concentration of the trivalent chromium may be in the range fromabout 0.1 molar (M) to about 5 M. The higher the concentration oftrivalent chromium, the higher the current density that can be appliedwithout resulting in a dendritic deposit, and consequently the fasterthe rate of crystalline chromium deposition that can be achieved.

The trivalent chromium bath may further include an organic additive suchas formic acid or a salt thereof, such as one or more of sodium formate,potassium formate, ammonium formate, calcium formate, magnesium formate,etc. Other organic additives, including amino acids such as glycine andthiocyanate may also be used to produce crystalline chromium depositsfrom trivalent chromium and their use is within the scope of oneembodiment of this invention. Chromium (III) formate, Cr(HCOO)₃, couldalso be used as a source of both trivalent chromium and formate.

The trivalent chromium bath may further include a source of nitrogen,which may be in the form of ammonium hydroxide or a salt thereof, or maybe a primary, secondary or tertiary alkyl amine, in which the alkylgroup is a C₁-C₆ alkyl. In one embodiment, the source of nitrogen isother than a quaternary ammonium compound. In addition to amines, aminoacids, hydroxy amines such as quadrol and polyhydric alkanolamines, canbe used as the source of nitrogen. In one embodiment of such nitrogensources, the additives include C₁-C₆ alkyl groups. In one embodiment,the source of nitrogen may be added as a salt, e.g., an amine salt suchas a hydrohalide salt.

As noted above, the crystalline chromium deposit may include carbon. Thecarbon source may be, for example, the organic compound such as formicacid or formic acid salt included in the bath. Similarly, thecrystalline chromium may include oxygen and hydrogen, which may beobtained from other components of the bath including electrolysis ofwater, or may also be derived from the formic acid or salt thereof, orfrom other bath components.

In addition to the chromium atoms in the crystalline chromium deposit,other metals may be co-deposited. As will be understood by those ofskill in the art, such metals may be suitably added to the trivalentchromium electroplating bath as desired to obtain various crystallinealloys of chromium in the deposit. Such metals include, but are notnecessarily limited to, Re, Cu, Fe, W, Ni, Mn, and may also include, forexample, P (phosphorus). In fact, all elements electrodepositable fromaqueous solution, directly or by induction, as described by Pourbaix orby Brenner, may be alloyed in this process. In one embodiment, thealloyed metal is other than aluminum. As is known in the art, metalselectrodepositable from aqueous solution include: Ag, As, Au, Bi, Cd,Co, Cr, Cu, Ga, Ge, Fe, In, Mn, Mo, Ni, P, Pb, Pd, Pt, Rh, Re, Ru, S,Sb, Se, Sn, Te, Tl, W and Zn, and inducible elements include B, C and N.As will be understood by those of skill in the art, the co-depositedmetal or atom is present in an amount less than the amount of chromiumin the deposit, and the deposit obtained thereby should be body-centeredcubic crystalline, as is the crystalline chromium deposit of the presentinvention obtained in the absence of such co-deposited metal or atom.

The trivalent chromium bath further comprises a pH of at least 4.0, andthe pH can range up to at least about 6.5. In one embodiment, the pH ofthe trivalent chromium bath is in the range from about 4.5 to about 6.5,and in another embodiment the pH of the trivalent chromium bath is inthe range from about 4.5 to about 6, and in another embodiment, the pHof the trivalent chromium bath is in the range from about 5 to about 6,and in one embodiment, the pH of the trivalent chromium bath is about5.5.

In one embodiment, the trivalent chromium bath is maintained at atemperature in the range from about 35° C. to about 115° C. or theboiling point of the solution, whichever is less, during the process ofelectrodepositing the crystalline chromium deposit of the presentinvention. In one embodiment, the bath temperature is in the range fromabout 45° C. to about 75° C., and in another embodiment, the bathtemperature is in the range from about 50° C. to about 65° C., and inone embodiment, the bath temperature is maintained at about 55° C.,during the process of electrodepositing the crystalline chromiumdeposit.

During the process of electrodepositing the crystalline chromium depositof the present invention, the electrical current is applied at a currentdensity of at least about 10 amperes per square decimeter (A/dm²). Inanother embodiment, the current density is in the range from about 10A/dm² to about 200 A/dm², and in another embodiment, the current densityis in the range from about 10 A/dm² to about 100 A/dm², and in anotherembodiment, the current density is in the range from about 20 A/dm² toabout 70 A/dm², and in another embodiment, the current density is in therange from about 30 A/dm² to about 60 A/dm², during theelectrodeposition of the crystalline chromium deposit from the trivalentchromium bath in accordance with the present invention.

During the process of electrodepositing the crystalline chromium depositof the present invention, the electrical current may be applied usingany one or any combination of two or more of direct current, pulsewaveform or pulse periodic reverse waveform.

Thus, in one embodiment, the present invention provides a process forelectrodepositing a crystalline chromium deposit on a substrate,including steps of:

providing an aqueous electroplating bath comprising trivalent chromium,formic acid or a salt thereof and at least one source of divalentsulfur, and substantially free of hexavalent chromium;

immersing a substrate in the electroplating bath; and

applying an electrical current to deposit a crystalline chromium depositon the substrate, wherein the chromium deposit is crystalline asdeposited.

In one embodiment, the crystalline chromium deposit obtained from thisprocess has a lattice parameter of 2.8895+/−0.0025 Å. In one embodiment,the crystalline chromium deposit obtained from this process has apreferred orientation (“PO”).

In another embodiment, the present invention provides a process forelectrodepositing a crystalline chromium deposit on a substrate,including steps of: providing an electroplating bath comprisingtrivalent chromium, formic acid and substantially free of hexavalentchromium;

immersing a substrate in the electroplating bath; and

-   -   applying an electrical current to deposit a crystalline chromium        deposit on the substrate, wherein the chromium deposit is        crystalline as deposited and the crystalline chromium deposit        has a lattice parameter of 2.8895+/−0.0025 Å. In one embodiment,        the crystalline chromium deposit obtained from this has a {111}        preferred orientation.

These processes in accordance with the invention may be carried outunder the conditions described herein, and in accordance with standardpractices for electrodeposition of chromium.

As noted above, a source of divalent sulfur is preferably provided inthe trivalent chromium electroplating bath. A wide variety of divalentsulfur-containing compounds can be used in accordance with the presentinvention.

In one embodiment, the source of divalent sulfur may include one or amixture of two or more of a compound having the general formula (I):

X¹—R¹—(S)_(n)—R²—X²  (I)

wherein in (I), X¹ and X² may be the same or different and each of X¹and X² independently comprise hydrogen, halogen, amino, cyano, nitro,nitroso, azo, alkylcarbonyl, formyl, alkoxycarbonyl, aminocarbonyl,alkylaminocarbonyl, dialkylaminocarbonyl, carboxyl (as used herein,“carboxyl” includes all forms of carboxyl groups, e.g., carboxylicacids, carboxylic alkyl esters and carboxylic salts), carboxylate,sulfonate, sulfinate, phosphonate, phosphinate, sulfoxide, carbamate,polyethoxylated alkyl, polypropoxylated alkyl, hydroxyl,halogen-substituted alkyl, alkoxy, alkyl sulfate ester, alkylthio,alkylsulfinyl, alkylsulfonyl, alkylphosphonate or alkylphosphinate,wherein the alkyl and alkoxy groups are C₁-C₆, or X¹ and X² takentogether may form a bond from R¹ to R², thus forming a ring containingthe R¹ and R² groups,

wherein R¹ and R² may be the same or different and each of R¹ and R²independently comprise a single bond, alkyl, allyl, alkenyl, alkynyl,cyclohexyl, aromatic and heteroaromatic rings, alkoxycarbonyl,aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, polyethoxylatedand polypropoxylated alkyl, wherein the alkyl groups are C₁-C₆, and

wherein n has an average value ranging from 1 to about 5.

In one embodiment, the source of divalent sulfur may include one or amixture of two or more of a compound having the general formula (IIa)and/or (IIb):

wherein in (IIa) and (IIb), R₃, R₄, R₅ and R₆ may be the same ordifferent and independently comprise hydrogen, halogen, amino, cyano,nitro, nitroso, azo, alkylcarbonyl, formyl, alkoxycarbonyl,aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, carboxyl,sulfonate, sulfinate, phosphonate, phosphinate, sulfoxide, carbamate,polyethoxylated alkyl, polypropoxylated alkyl, hydroxyl,halogen-substituted alkyl, alkoxy, alkyl sulfate ester, alkylthio,alkylsulfinyl, alkylsulfonyl, alkylphosphonate or alkylphosphinate,wherein the alkyl and alkoxy groups are C₁-C₆,

wherein X represents carbon, nitrogen, oxygen, sulfur, selenium ortellurium and in which m ranges from 0 to about 3,

wherein n has an average value ranging from 1 to about 5, and

wherein each of (IIa) or (IIb) includes at least one divalent sulfuratom.

In one embodiment, the source of divalent sulfur may include one or amixture of two or more of a compound having the general formula (IIIa)and/or (IIIb):

wherein, in (IIIa) and (IIIb), R₃, R₄, R₅ and R₆ may be the same ordifferent and independently comprise hydrogen, halogen, amino, cyano,nitro, nitroso, azo, alkylcarbonyl, formyl, alkoxycarbonyl,aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, carboxyl,sulfonate, sulfinate, phosphonate, phosphinate, sulfoxide, carbamate,polyethoxylated alkyl, polypropoxylated alkyl, hydroxyl,halogen-substituted alkyl, alkoxy, alkyl sulfate ester, alkylthio,alkylsulfinyl, alkylsulfonyl, alkylphosphonate or alkylphosphinate,wherein the alkyl and alkoxy groups are C₁-C₆,

wherein X represents carbon, nitrogen, sulfur, selenium or tellurium andin

which n ranges from 0 to about 3,

wherein n has an average value ranging from 1 to about 5, and

wherein each of (IIIa) or (IIIb) includes at least one divalent sulfuratom.

In one embodiment, in any of the foregoing sulfur containing compounds,the sulfur may be replaced by selenium or tellurium. Exemplary seleniumcompounds include seleno-DL-methionine, seleno-DL-cystine, otherselenides, R—Se—R′, diselenides, R—Se—Se—R′ and selenols, R—Se—H, whereR and R′ independently may be an alkyl or aryl group having from 1 toabout 20 carbon atoms, which may include other heteroatoms, such asoxygen or nitrogen, similar to those disclosed above for sulfur.Exemplary tellurium compounds include ethoxy and methoxy telluride,Te(OC₂H₅)₄ and Te(OCH₃)₄.

As will be understood, the substituents used are preferably selected sothat the compounds thus obtained remain soluble in the electroplatingbaths of the present invention.

Comparative Examples Hexavalent Chromium

In Table 1 comparative examples of various aqueous hexavalent chromicacid containing electrolytes that produce functional chromium depositsare listed, the crystallographic properties of the deposit tabulated,and reported elemental composition based upon C, O, H, N and S analysis.

TABLE 1 Hexavalent chromium based electrolytes for functional chromiumH1 H2 H3 H4 H5 H6 CrO₃ (M) 2.50 2.50 2.50 2.50 2.50 8.00 H₂SO₄ (M) 0.0260.015 0.029 MgSiF₆ (M) 0.02 CH₂(SO₃Na)₂ (M) 0.015 KIO₃ (M) 0.016 0.009HO₃SCH₂CO₂H (M) 0.18 HCl (M) 11.7N 0.070 H₂O to 1 L to 1 L to 1 L to 1 Lto 1 L to 1 L Current Density 30 20 45 50 50 62 (A/dm²) Temperature, °C. 55 55 50 60 55 50 Cathodic efficiency, % 2-7 10-15 15-25 20-30 35-4055-60 Lattice(s) BCC BCC BCC BCC BCC- BCC SC Grain Preferred Random(222) (222) (222) (110) Random Orientation PO (211) PO PO PO Latticeparameter 2.883 2.882 2.883 2.881 2.882 2.886 as deposited Bulk [C] at %— — 0.04 0.06 Bulk [H] at % 0.055 0.078 0.076 0.068 Bulk [O₂] at % 0.360.62 0.84 0.98 Bulk [S] at % — — 0.04 0.12

In Table 2 comparative examples of trivalent chromium process solutionsdeemed by the Ecochrome project to be the best available technology aretabulated. The Ecochrome project was a multiyear European Unionsponsored program (G1RD CT-2002-00718) to find an efficient and highperformance hard chromium alternative based upon trivalent chromium(see, Hard Chromium Alternatives Team (HCAT) Meeting, San Diego, Calif.,Jan. 24-26, 2006). The three processes are from Cidetec, a consortiumbased in Spain; ENSME, a consortium based in France; and, Musashi, aconsortium based in Japan. In this table, where no chemical formula isspecifically listed, the material is believed to be proprietary in thepresentations from which these data were obtained, and is not available.

TABLE 2 Best available known technology for functional trivalentchromium processes from the Ecochrome project. EC1 EC2 EC3 (Cidetec)(ENSME) (Musashi) Cr(III) (M) 0.40 1.19 CrCl₃•6H₂O (M) 1.13 fromCr(OH)₃ + 3HCl H₂NCH₂CO₂H (M) 0.67 Ligand 1 (M) 0.60 Ligand 2 (M) 0.30Ligand 3 (M) 0.75 H₃BO₃ (M) 0.75 Conductivity salts 2.25 (M) HCO₂H (M)0.19 NH₄Cl (M) 0.19 2.43 H₃BO₃ (M) 0.08 0.42 AlCl₃•6H₂O (M) 0.27Surfactant ml/L 0.225 0.2 pH   2-2.3 ~0.1 ~0.3 Temp (° C.) 45-50 50 50Current density 20.00 70.00 40.00 A/dm² Cathodic efficiency 10% ~27% 13%Structure as plated amorphous amorphous amorphous Orientation NA NA NAIn the Table 2 comparative examples, the EC3 example contains aluminumchloride. Other trivalent chromium solutions containing aluminumchloride have been described. Suvegh et al. (Journal ofElectroanalytical Chemistry 455 (1998) 69-73) use an electrolytecomprising 0.8 M [Cr(H₂0)₄Cl₂]Cl.2H₂0, 0.5 M NH₄Cl, 0.5 M NaCl, 0.15 MH₃BO₃, 1 M glycine, and 0.45 M AlCl₃, pH not described. Hong et al.(Plating and Surface Finishing, March 2001) describe an electrolytecomprising mixtures of carboxylic acids, a chromium salt, boric acid,potassium chloride, and an aluminum salt, at pH 1-3). Ishida et al.(Journal of the Hard Chromium Platers Association of Japan 17, No. 2,Oct. 31, 2002) describe solutions comprising 1.126 M[Cr(H₂0)₄Cl₂]Cl.2H₂0, 0.67 M glycine, 2.43 M NH₄Cl, and 0.48 M H₃BO₃ towhich varying amounts of AlCl₃.6H₂O, from 0.11 to 0.41M were added; pHwas not described. Of these four references disclosing aluminum chloridein the trivalent chromium bath, only Ishida et al. contends that thechromium deposit is crystalline, stating that crystalline depositsaccompany increasing concentrations of AlCl₃. However, repeated attemptsby the present inventors to replicate the experiment and producecrystalline deposits have failed. It is believed that an importantexperimental variable is not described by Ishida et al. Therefore, it isconsidered that Ishida et al. fails to teach how to make a reliablyconsistent crystalline chromium deposit.

In Table 3 various aqueous (“T”) trivalent chromium-containingelectrolytes and one ionic liquid (“IL”) trivalent chromium-containingelectrolyte, all of which can produce chromium deposits in excess of onemicron thickness, are listed and the crystallographic properties of thedeposit tabulated.

TABLE 3 Trivalent chromium based electrolytes for functional chromium T1T2 T3 T4 T5 T6 T7 IL1 MW Cr(OH)SO₄•Na₂SO₄ 0.39 0.39 0.39 0.55 0.39 307(M) KCl (M) 3.35 74.55 H₃BO₃ (M) 1.05 61.84 HCO₂ ⁻K⁺ 0.62 84.1 (M)CrCl₃•6H2O 1.13 2.26 266.4 (M) Cr(HCO₂)₃ 0.38 187 (M) CH₂OHCH₂ 2.13139.5 N⁺(CH₃)₃Cl⁻ (M) NH₄CHO₂ 3.72 5.55 63.1 (M) LiCl (M) 2.36 42.4HCO₂H (M) 3.52 3.03 3.52 0.82 4.89 46.02 NH₄OH (M) 5.53 4.19 5.53 35(NH₄)₂SO₄ 0.61 0.61 1.18 132.1 (M) NH₄Cl (M) 0.56 0.56 1.87 0.56 0.5653.5 NH₄Br (M) 0.10 0.10 0.51 0.10 0.10 0.10 97.96 Na₄P₂O₇•10H₂O 0.0340.034 0.034 446 (M) KBr (M) 0.042 119 H₂O to 1 L to 1 L to 1 L to 1 L to1 L to 1 L to 1 L none 18 pH 0.1-3 0.1-3 0.1-3 0.1-3 0.1-3 0.1-3 0.1-3NA Current 12.4 20 20 20 20 50 80 density (A/dm²) Temp. ° C. 45 45 45 4545 45 45 80 Cathodic 25% 15% 15% 15% 15% 30% ~10% eff. Lattice(s) Amor.Amor. Amor. Amor. Amor. Amor. NA SC Grain Pref. NA NA NA NA NA Pwdr PwdrRndm Orientation Lattice 2.882 2.884 2.882 2.886 2.883 NA NA — parameterafter anneal 4 hr./191° C. Organic Amor. xtal. xtal. xtal. xtal. xtal.xtal. — additives pH > 4 Grain (111), (111), (111), (111), (111), (111),Orientation Rndm Rndm Rndm Rndm Rndm Rndm Electrolyte + Amor. xtal.xtal. xtal. xtal. xtal. xtal. AlCl₃•6H₂0 0.62M, pH < 3 (In Table 3: Pwdr= powder; Amor. = amorphous; rndm = random; NA = not applicable; SC =simple cubic; xtal. = crystalline)

In Table 4 the various deposits from Tables 1, 2 and 3 are comparedusing standard test methods frequently used for evaluation ofas-deposited functional chromium electrodeposits. From this table it canbe observed that amorphous deposits, and deposits that are not BCC (bodycentered cubic) do not pass all the necessary initial tests.

TABLE 4 Comparison of test results on as deposited functional chromiumfrom electrolytes in tables 1-3 Macro- Hardness Cracks Grind crack afterVickers from Electrolyte Structure Orientation Appearance test heating(100 g) indentation? H1 BCC random powdery fail Yes — — H2 BCC (222)lustrous pass No 900 No H3 BCC (222)(211) lustrous pass No 950 No H4 BCC(222) lustrous pass No 950 No H5 BCC + SC (222)(110) lustrous fail No900 No H6 BCC random. lustrous fail No 960 Yes EC1 amor. NA lustrousfail Yes 845-1000 Yes EC2 amor. NA lustrous fail Yes 1000  Yes EC3 amor.NA lustrous fail Yes — Yes T1 amor. NA lustrous fail Yes 1000  Yes T2amor. NA lustrous fail Yes 950 Yes T3 amor. NA lustrous fail Yes 950 YesT4 amor. NA lustrous fail Yes 900 Yes T5 amor. NA lustrous fail Yes1050  No T6 amor. NA lustrous fail Yes 950 Yes T7 powdery — — — — — —IL1 SC random black fail Yes — — particulate

In accordance with industrial requirements for replacement of hexavalentchromium electrodeposition baths, the deposits from trivalent chromiumelectrodeposition baths must be crystalline to be effective and usefulas a functional chromium deposit. It has been found that certainadditives can be used together with adjustments in the process variablesof the electrodeposition process to obtain a desirably crystallinechromium deposit from a trivalent chromium bath that is substantiallyfree of hexavalent chromium. Typical process variables include currentdensity, solution temperature, solution agitation, concentration ofadditives, manipulation of the applied current waveform, and solutionpH. Various tests may be used to accurately assess the efficacy of aparticular additive, including, e.g., X-ray diffraction (XRD) (to studythe structure of the chromium deposit), X-ray photoelectron spectroscopy(XPS)(for determination of components of the chromium deposit, greaterthan about 0.2-0.5 wt. %), elastic recoil determination (ERD) (fordetermination of hydrogen content), and electron microscopy (fordetermination of physical or morphological characteristics such ascracking).

In the prior art, it has been generally and widely considered thatchromium deposition from trivalent chromium baths must occur at pHvalues less than about 2.5. However, there are isolated trivalentchromium plating processes, including brush plating processes, wherehigher pH's have been used, although the higher pH's used in these brushplating solutions do not result in a crystalline chromium deposit.Therefore, in order to assess the efficacy of various additives, stable,high pH electrolytes were tested as well as the commonly accepted low pHelectrolytes.

TABLE 5 Additives inducing crystallization from trivalent chromium bathT2. Concentration Range T2 pH 2.5: T2 pH 4.2: Additive AddedCrystalline? Crystalline? Methionine 0.1, 0.5, 1.0, 1.5 g/L no no, yes,yes, na Cystine 0.1, 0.5, 1.0, 1.5 g/L no yes, yes, yes, yesThiomorpholine 0.1, 0.5, 1, 1.5, 2, 3 mL/L no no, no, yes, yes, yes, yesThiodipropionic Acid 0.1, 0.5, 1.0, 1.5 g/L no no, yes, yes, yesThiodiethanol 0.1, 0.5, 1.0, 1.5 g/L no no, yes, yes, yes Cysteine 0.1,1, 2.0, 3.0 g/L no yes, yes, yes, yes, Allyl Sulfide 0.5, 1.0, 1.5 mL/Lno no, yes, yes, na Thiosalicylic Acid 0.5, 1, 1.5 no yes, yes, yes3,3′-dithio 1, 2, 5, 10 g/L no yes, yes, yes, yes, dipropanoic acidTetrahydrothiophene 0.5, 1.0, 1.5 mL/L no no, yes, yes

From the data shown in Table 5 it is apparent that compounds that havedivalent sulfur in their structure induce crystallization when chromiumis electrodeposited from a trivalent chromium solution, at about theabove-stated concentrations and when the pH of the bath is greater thanabout 4, in which the chromium crystals have the lattice parameter of2.8895+/−0.0025 Å, in accordance with the present invention. In oneembodiment, other divalent sulfur compounds can be used in the bathsdescribed herein to electrodeposit crystalline chromium having thelattice parameter of the present invention. In one embodiment, compoundshaving sulfur, selenium or tellurium, when used as described herein,also induce crystallization of chromium. In one embodiment, the seleniumand tellurium compounds correspond to the above-identified sulfurcompounds, and like the sulfur compounds, result in theelectrodeposition of crystalline chromium having a lattice parameter of2.8895+/−0.0025 Å.

To further illustrate the induction of crystallization, studies oncrystallization inducing additives using electrolyte T3 at pH 5.5 andtemperature 50° C. with identical cathode current densities of 40 A/dm²and plating times of thirty minutes using brass substrate are reportedin Table 6. After plating is complete the coupons are examined usingX-ray diffraction, X-ray induced X-ray fluorescence for thicknessdetermination, and electron induced X-ray fluorescence with an energydispersive spectrophotometer to measure sulfur content. Table 6summarizes the data. The data may suggest that it is not only thepresence of a divalent sulfur compound in the solution at aconcentration exceeding a threshold concentration that inducescrystallization but the presence of sulfur in the deposit, as well.

TABLE 6 Induction of sulfur from various divalent sulfur additives andthe effects on as-plated crystallization of Cr for Cr + 3 solution, andplating rate. [S] wt Additive Thickness % in Additive per L Crystalline(um) deposit Methionine 0.1 g no 3.13 2.1 0.5 g yes 2.57 4.3 1.0 g yes4.27 3.8 1.5 g (insoluble) 7.17 2.6 Cystine 0.1 g yes 1.62 3.9 0.5 g yes0.75 7.1 1.0 g yes 1.39 9.3 1.5 g yes 0.25 8.6 Thiomorpholine 0.1 mL no6.87 1.7 0.5 mL no 11.82 3.9 1 mL yes 7.7 5.9 1.5 mL yes 2.68 6.7 2 mLyes 4.56 7.8 3 mL yes 6.35 7.1 Thiodipropionic Acid 0.1 g no 6.73 1 0.5g yes 4.83 3.5 1.0 g yes 8.11 1.8 1.5 g yes 8.2 3.1 Thiodiethanol 0.1 mLno 4.88 0.8 0.5 mL yes 5.35 4 1.0 mL yes 6.39 4 1.5 mL yes 3.86 4.9Cysteine 0.1 g yes 2.08 5.1 1.0 g yes 1.3 7.5 2.0 g yes 0.35 8.3 3.0 gyes 0.92 9.7 Allyl Sulfide 0.1 mL no 6.39 1.3 (oily) 0.5 mL yes 4.06 3.41.0 mL yes 1.33 4.9 1.5 mL (insoluble) 5.03 2.6 Thiosalicylic Acid 0.5 gyes 2.09 5.8 1.0 g yes 0.52 5.5 1.5 g yes 0.33 7.2 1.5 g yes 0.33 7.23,3′thiodipropanoic acid 1 g yes 7.5 5.9 2 g yes 6 6.1 5 g yes 4 6 10 gyes 1 6.2 (S content determined by EDS) (“(insoluble)” means theadditive was saturated at the given concentration)

The following Table 7 provides additional data relating toelectroplating baths of trivalent chromium in accordance with thepresent invention.

TABLE 7 Representative formulations for production of as-depositedcrystalline Cr from solutions of Cr + 3. pH-° C.- Cathode preferredProcess Electrolyte Additive A/dm² Efficiency orientation H_(v) [C] [S][N]] P1 T2 4 ml/L thio- 5.5-50-40  5-10% (222) 900-980 3.3 1.57 0.6morpholine P2 T2 3 ml/L thio- 5.5-50-40 10% Random — 3.0 1.4 0.6diethanol and (222) P3 T2 1 g/L l- 5.5-50-40  5% Random — cysteine and(222) P4 T5 4 ml/L thio- 5.5-50-40  5-10% (222) 900-980 morpholine P5 T53 ml/L thio- 5.5-50-40 10% Random — diethanol and (222) P6 T5 1 g/L l-5.5-50-40  5% Random — cysteine and (222) P7 T5 4 ml/L thio- 5.5-50-4015% (222) 900-980 morpholine P8 T5 3 ml/L thio- 5.5-50-40 10-12% Random— diethanol and (222) P9 T5 1 g/L l- 5.5-50-40 7-9% Random — cysteineand (222) P10 T5 2 g/L 5.5-50-40 10-12% (222) 940-975 5.5 1.8 1.3thiosalicylic acid P11 T5 2 g/L 3,3′- 5.5-50-40 12-15% (222) 930-980 4.92.1 1.1 dithiodipropanoic acid

The above examples are prepared with direct current and without the useof complex cathodic waveforms such as pulse or periodic reverse pulseplating, although such variations on the applied electrical current arewithin the scope of the present invention. All of the examples in Table7 that are crystalline have a lattice constant of 2.8895+/−0.0025 Å, asdeposited.

In a further example of the utility of this invention pulse depositionsare performed using simple pulse waveforms generated with a PrincetonApplied Research Model 273A galvanostat equipped with a power boosterinterface and a Kepco bipolar +/−10 A power supply, using process P1,with and without thiomorpholine. Pulse waveforms are square wave, 50%duty cycle, with sufficient current to produce a 40 A/dm² currentdensity overall. The frequencies employed are 0.5 Hz, 5 Hz, 50 Hz, and500 Hz. At all frequencies the deposits from process P1 withoutthiomorpholine are amorphous while the deposits from process P1 withthiomorpholine are crystalline as deposited.

In a further example of the utility of this invention pulse depositionsare performed using simple pulse waveforms generated with a PrincetonApplied Research Model 273A galvanostat equipped with a power boosterinterface and a Kepco bipolar +/−10 A power supply, using process P1,with and without thiomorpholine. Pulse waveforms are square wave, 50%duty cycle, with sufficient current to produce a 40 A/dm² currentdensity overall. The frequencies employed are 0.5 Hz, 5 Hz, 50 Hz, and500 Hz. At all frequencies the deposits from process P1 withoutthiomorpholine are amorphous while the deposits from process P1 withthiomorpholine are crystalline as deposited, and have a lattice constantof 2.8895+/−0.0025 Å.

Similarly the electrolyte T5 is tested with and without thiosalicylicacid at a concentration of 2 g/L using a variety of pulse waveformshaving current ranges of 66-109 A/dm² with pulse durations from 0.4 to200 ms and rest durations of 0.1 to 1 ms including periodic reversewaveforms with reverse current of 38-55 A/dm² and durations of 0.1 to 2ms. In all cases, without thiosalicylic acid the deposit is amorphous,with thiosalicylic acid the deposit is crystalline, and has a latticeconstant of 2.8895+/−0.0025 Å.

In one embodiment, the crystalline chromium deposits are homogeneous,without the deliberate inclusion of particles, and have a latticeconstant of 2.8895+/−0.0025 Å. For example, particles of alumina,Teflon, silicon carbide, tungsten carbide, titanium nitride, etc. may beused with the present invention to form crystalline chromium depositsincluding such particles within the deposit. Use of such particles withthe present invention is carried out substantially in the same manner asis known from prior art processes.

The foregoing examples use anodes of platinized titanium. However, theinvention is in no way limited to the use of such anodes. In oneembodiment, a graphite anode may be used as an insoluble anode. Inanother embodiment, a soluble chromium or ferrochromium anodes may beused.

In one embodiment, the anodes may be isolated from the bath. In oneembodiment, the anodes may be isolated by use of a fabric, which may beeither tightly knit or loosely woven. Suitable fabrics include thoseknown in the art for such use, including, e.g., cotton andpolypropylene, the latter available from Chautauqua Metal FinishingSupply, Ashville, N.Y. In another embodiment, the anode may be isolatedby use of anionic or cationic membranes, for example, such asperfluorosulfonic acid membranes sold under the tradenames NAFION®(DuPont), ACIPLEX® (Asahi Kasei), FLEMION® (Asahi Glass) or otherssupplied by Dow or by Membranes International Glen Rock, N.J. In oneembodiment, the anode may be placed in a compartment, in which thecompartment is filled with an acidic, neutral, or alkaline electrolytethat differs from the bulk electrolyte, by an ion exchange means such asa cationic or anionic membrane or a salt bridge.

FIG. 1 includes three X-ray diffraction patterns (Cu k alpha) ofcrystalline chromium deposited in accordance with an embodiment of thepresent invention and with hexavalent chromium of the prior art. TheseX-ray diffraction patterns include, at the bottom and the center, acrystalline chromium deposited from trivalent chromium electrolyte T5with 2 g/L (bottom) and 10 g/L (center) of 3,3′-dithiodipropanoic (DTDP)acid in the trivalent chromium bath, respectively. Each of these sampleswere deposited with a similar deposition time and current density. Thetop sample, in contrast, is a conventional chromium deposit fromhexavalent electrolyte H4 (as described above). As shown in the top andbottom scans, for both the hexavalent chromium and the 2 g/l DTDP case,the absence of brass substrate peaks (identified by (*) for the centerscan; see also FIG. 9 and text relating thereto) indicate thickdeposits, greater than ˜20 microns (the penetration depth of Cu k alpharadiation through chromium). In contrast, the presence of the brasspeaks in the 10 g/L DTDP case shows that excess DTDP may diminishcathodic efficiency. In both DTDP cases however, the strong and broad(222) reflection demonstrates strong {111} preferred orientation ispresent and that the continuously diffracting domains of the chromium,generally thought to correlate with grain size, are very small, and aresimilar to chrome from hexavalent process H4.

FIG. 2 is a typical X-ray diffraction pattern (Cu k alpha) of amorphouschromium from a trivalent chromium bath of the prior art. As shown inFIG. 2, there are no sharp peaks corresponding to regularly occurringpositions of atoms in the structure, which would be observed if thechromium deposit were crystalline.

FIG. 3 is a series of typical X-ray diffraction pattern (Cu k alpha)showing the progressive effect of annealing an amorphous chromiumdeposit from a trivalent chromium bath of the prior art, containing nosulfur. In FIG. 3 there is shown a series of X-ray diffraction scans,starting at the lower portion and proceeding upward in FIG. 3, as thechromium deposit is annealed for longer and longer periods of time. Asshown in FIG. 3, initially, the amorphous chromium deposit results in anX-ray diffraction pattern similar to that of FIG. 2, but with continuedannealing, the chromium deposit gradually crystallizes, resulting in apattern of sharp peaks corresponding to the regularly occurring atoms inthe ordered crystal structure. The lattice parameter of the annealedchromium deposit is in the 2.882 to 2.885 range, although the quality ofthis series is not good enough to measure accurately.

FIG. 4 is a series of electron photomicrographs showing themacrocracking effect of annealing an initially amorphous chromiumdeposit from a trivalent chromium bath of the prior art. In thephotomicrograph labeled “As deposited amorphous chromium” the chromiumlayer is the lighter-colored layer deposited on the mottled-appearingsubstrate. In the photomicrograph labeled “1 h at 250° C.”, afterannealing at 250° C. for one hour, macrocracks have formed, while thechromium deposit crystallizes, the macrocracks extend through thethickness of the chromium deposit, down to the substrate. In this andthe subsequent photomicrographs, the interface between the chromiumdeposit and the substrate is the faint line running roughlyperpendicular to the direction of propagation of the macrocracks, and ismarked by the small black square with “P1” within. In thephotomicrograph labeled “1 h at 350° C.”, after annealing at 350° C. forone hour, larger and more definite macrocracks have formed (compared tothe “1 h at 250° C.” sample), while the chromium deposit crystallizes,the macrocracks extend through the thickness of the chromium deposit,down to the substrate. In the photomicrograph labeled “1 h at 450° C.”,after annealing at 450° C. for one hour, the macrocracks have formed andare larger than the lower temperature samples, while the chromiumdeposit crystallizes, the macrocracks extend through the thickness ofthe chromium deposit, down to the substrate. In the photomicrographlabeled “1 h at 550° C.”, after annealing at 550° C. for one hour, themacrocracks have formed and appear to be larger yet than the lowertemperature samples, while the chromium deposit crystallizes, themacrocracks extend through the thickness of the chromium deposit, downto the substrate.

FIG. 5 shows a typical X-ray diffraction pattern (Cu k alpha) of acrystalline as-deposited chromium deposit in accordance with the presentinvention. As shown in FIG. 5, the X-ray diffraction peaks are sharp andwell defined, showing that the chromium deposit is crystalline, inaccordance with the invention.

FIG. 6 shows typical X-ray diffraction patterns (Cu k alpha) ofcrystalline chromium deposits in accordance with the present invention.The middle two X-ray diffraction patterns shown in FIG. 6 demonstratestrong (222) peaks indicating the {111} preferred orientation (PO)similar to that observed with crystalline chromium deposited from ahexavalent chromium bath. The top and bottom X-ray diffraction patternsshown in FIG. 6 include (200) peaks indicating preferred orientationsobserved for other crystalline chromium deposits.

FIG. 7 is a graphical chart illustrating how the concentration of sulfurin one embodiment of a chromium deposit relates to the crystallinity ofthe chromium deposit. In the graph shown in FIG. 7, if the deposit iscrystalline, the crystallinity axis is assigned a value of one, while ifthe deposit is amorphous, the crystallinity axis is assigned a value ofzero. Thus, in the embodiment shown in FIG. 7, where the sulfur contentof the chromium deposit ranges from about 1.7 wt. % to about 4 wt. %,the deposit is crystalline, while outside this range, the deposit isamorphous. It is noted in this regard, that the amount of sulfur presentin a given crystalline chromium deposit can vary. That is, in someembodiments, a crystalline chromium deposit may contain, for example,about 1 wt. % sulfur and be crystalline, and in other embodiments, withthis sulfur content, the deposit would be amorphous (as in FIG. 7). Inother embodiments, a higher sulfur content, for example, up to about 10wt. %, might be found in a chromium deposit that is crystalline, whilein other embodiments, if the sulfur content is greater than 4 wt. %, thedeposit may be amorphous. Thus, sulfur content is important, but notcontrolling and not the only variable affecting the crystallinity of thetrivalent-derived chromium deposit.

FIG. 8 is a graphical chart comparing the crystal lattice parameter, inAngstroms (Å) for a crystalline chromium deposit in accordance with thepresent invention with crystalline chromium deposits from hexavalentchromium baths and annealed amorphous-as deposited chromium deposits. Asshown in FIG. 8, the lattice parameter of a crystalline chromium depositin accordance with the present invention is significantly greater anddistinct from the lattice parameter of pyrometallurgically derivedchromium (“PyroCr”), is significantly greater and distinct from thelattice parameters of all of the hexavalent chromium deposits(“H1”-“H6”), and is significantly greater and distinct from the latticeparameters of the annealed amorphous-as-deposited chromium deposits(“T1(350° C.)”, “T1(450° C.)” and “T1(550° C.)”). The difference betweenthe lattice parameters of the trivalent crystalline chromium deposits ofthe present invention and the lattice parameters of the other chromiumdeposits, such as those illustrated in FIG. 8, is statisticallysignificant, at least at the 95% confidence level, according to thestandard Student's ‘t’ test.

FIG. 9 is a typical X-ray diffraction pattern (Cu k alpha) showing theprogressive effect of increasing amounts of thiosalicylic acid showingthe reliably consistent (222) reflection, {111} preferred orientation,crystalline chromium deposit from a trivalent chromium bath inaccordance with an embodiment of the present invention. In FIG. 9,crystalline chromium was deposited on brass substrates (peaks from thebrass identified by (*)) from trivalent chromium electrolyte T5 (asdescribed above) electrolyzed at 10 amps per liter (A/L) with nominal2-6 g/L thiosalicylic acid present to an excess of 140 AH/Ldemonstrating reliably consistent (222) reflection, {111} preferredorientation, deposits. The samples were taken at ˜14 AH intervals.

In one embodiment, the cathodic efficiency ranges from about 5% to about80%, and in one embodiment, the cathodic efficiency ranges from about10% to about 40%, and in another embodiment, the cathodic efficiencyranges from about 10% to about 30%.

In another embodiment additional alloying of the crystalline chromiumelectrodeposit, in which the chromium has a lattice constant of2.8895+/−0.0025 Å, may be performed using ferrous sulfate and sodiumhypophosphite as sources of iron and phosphorous with and without theaddition of 2 g/L thiosalicylic acid. Additions of 0.1 g/L to 2 g/L offerrous ion to electrolyte T7 result in alloys containing 2 to 20% iron.The alloys are amorphous without the addition of thiosalicylic acid.Additions of 1 to 20 g/L sodium hypophosphite resulted in alloyscontaining 2 to 12% phosphorous in the deposit. The alloys wereamorphous unless thiosalicylic acid is added.

In another embodiment, crystalline chromium deposits having a latticeconstant of 2.8895+/−0.0025 Å are obtained from electrolyte T7 with 2g/L thiosalicylic acid agitated using ultrasonic energy at a frequencyof 25 kHz and 0.5 MHz. The resulting deposits are crystalline, having alattice constant of 2.8895+/−0.0025 Å, bright, and there is nosignificant variation in deposition rate regardless of the frequencyused.

It is noted that, throughout the specification and claims, the numericallimits of the disclosed ranges and ratios may be combined, and aredeemed to include all intervening values. Thus, for example, whereranges of 1-100 and 10-50 are specifically disclosed, ranges of 1-10,1-50, 10-100 and 50-100 are deemed to be within the scope of thedisclosure, as are the intervening integral values. Furthermore, allnumerical values are deemed to be preceded by the modifier “about”,whether or not this term is specifically stated. Furthermore, when thechromium deposit is electrodeposited from a trivalent chromium bath asdisclosed herein in accordance with the present invention, and thethus-formed deposit is stated herein as being crystalline, it is deemedto have a lattice constant of 2.8895+/−0.0025 Å, whether or not thislattice constant is specifically stated. Finally, all possiblecombinations of disclosed elements and components are deemed to bewithin the scope of the disclosure, whether or not specificallymentioned.

While the principles of the invention have been explained in relation tocertain particular embodiments, and are provided for purposes ofillustration, it is to be understood that various modifications thereofwill become apparent to those skilled in the art upon reading thespecification. Therefore, it is to be understood that the inventiondisclosed herein is intended to cover such modifications as fall withinthe scope of the appended claims. The scope of the invention is limitedonly by the scope of the claims.

1. An electrodeposition bath for electrodepositing a crystalline chromium deposit, comprising: a source of trivalent chromium having a concentration of least 0.1 molar and being substantially free of added hexavalent chromium; an organic additive; a source of divalent sulfur; a pH in the range from 4 to about 6.5; an operating temperature in the range from about 35° C. to about 95° C.; and a source of electrical energy applied between an anode and a cathode immersed in the electrodeposition bath.
 2. The electrodeposition bath of claim 1 wherein the source of divalent sulfur comprises one or a mixture of two or more of a compound having the general formula (I): X¹—R¹—(S)_(n)—R²—X²  (I) wherein in (I), X¹ and X² may be the same or different and each of X¹ and X² independently comprise hydrogen, halogen, amino, cyano, nitro, nitroso, azo, alkylcarbonyl, formyl, alkoxycarbonyl, aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, carboxyl, sulfonate, sulfinate, phosphonate, phosphinate, sulfoxide, carbamate, polyethoxylated alkyl, polypropoxylated alkyl, hydroxyl, halogen-substituted alkyl, alkoxy, alkyl sulfate ester, alkylthio, alkylsulfinyl, alkylsulfonyl, alkylphosphonate or alkylphosphinate, wherein the alkyl and alkoxy groups are C₁-C₆, or X¹ and X² taken together may form a bond from R¹ to R², wherein R¹ and R² may be the same or different and each of R¹ and R² independently comprise a single bond, alkyl, allyl, alkenyl, alkynyl, cyclohexyl, aromatic and heteroaromatic rings, alkoxycarbonyl, aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, polyethoxylated and polypropoxylated alkyl, wherein the alkyl groups are C₁-C₆, and wherein n has an average value ranging from 1 to about
 5. 3. The electrodeposition bath of claim 1 wherein the source of divalent sulfur comprises one or a mixture of two or more of a compound having the general formula (IIa) and/or (IIb):

wherein in (IIa) and (IIb), R₃, R₄, R₅ and R₆ may be the same or different and independently comprise hydrogen, halogen, amino, cyano, nitro, nitroso, azo, alkylcarbonyl, formyl, alkoxycarbonyl, aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, carboxyl, sulfonate, sulfinate, phosphonate, phosphinate, sulfoxide, carbamate, polyethoxylated alkyl, polypropoxylated alkyl, hydroxyl, halogen-substituted alkyl, alkoxy, alkyl sulfate ester, alkylthio, alkylsulfinyl, alkylsulfonyl, alkylphosphonate or alkylphosphinate, wherein the alkyl and alkoxy groups are C₁-C₆, wherein X represents carbon, nitrogen, oxygen, sulfur, selenium or tellurium and in which m ranges from 0 to about 3, wherein n has an average value ranging from 1 to about 5, and wherein each of (IIa) or (IIb) includes at least one divalent sulfur atom.
 4. The electrodeposition bath of claim 1 wherein the source of divalent sulfur comprises one or a mixture of two or more of a compound having the general formula (IIIa) and/or (IIIb):

wherein, in (IIIa) and (IIIb), R₃, R₄, R₅ and R₆ may be the same or different and independently comprise hydrogen, halogen, amino, cyano, nitro, nitroso, azo, alkylcarbonyl, formyl, alkoxycarbonyl, aminocarbonyl, alkylaminocarbonyl, dialkylaminocarbonyl, carboxyl, sulfonate, sulfinate, phosphonate, phosphinate, sulfoxide, carbamate, polyethoxylated alkyl, polypropoxylated alkyl, hydroxyl, halogen-substituted alkyl, alkoxy, alkyl sulfate ester, alkylthio, alkylsulfinyl, alkylsulfonyl, alkylphosphonate or alkylphosphinate, wherein the alkyl and alkoxy groups are C₁-C₆, wherein X represents carbon, nitrogen, sulfur, selenium or tellurium and in which m ranges from 0 to about 3, wherein n has an average value ranging from 1 to about 5, and wherein each of (IIIa) or (IIIb) includes at least one divalent sulfur atom.
 5. The electrodeposition bath of claim 1 further comprising a source of nitrogen.
 6. The electrodeposition bath of claim 5 wherein the source of nitrogen comprises ammonium hydroxide or a salt thereof, a primary, secondary or tertiary alkyl amine, in which the alkyl group is a C₁-C₆ alkyl, an amino acid, a hydroxy amine, or a polyhydric alkanolamines, wherein alkyl groups in the source of nitrogen comprise C₁-C₆ alkyl groups.
 7. The electrodeposition bath of claim 1 wherein the source of electrical energy is capable of providing a current density of at least 10 A/dm² based on an area of substrate to be plated.
 8. The electrodeposition bath of claim 1 wherein the source of electrical energy is capable of applying one or more of direct current, pulse waveform or pulse periodic reverse waveform.
 9. The electrodeposition bath of claim 1 wherein when operated the bath deposits a functional chromium deposit that is crystalline as deposited.
 10. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit has a lattice parameter of 2.8895+/−0.0025 Å.
 11. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit has a {111} preferred orientation.
 12. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit further comprises carbon, nitrogen and sulfur in the chromium deposit.
 13. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit comprises from about 1 wt. % to about 10 wt. % sulfur.
 14. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit comprises from about 0.1 to about 5 wt % nitrogen.
 15. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit comprises an amount of carbon less than that amount which renders the chromium deposit amorphous.
 16. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit comprises from about 1.7 wt. % to about 4 wt. % sulfur, from about 0.1 wt. % to about 3 wt. % nitrogen, and from about 0.1 wt. % to about 10 wt. % carbon.
 17. The electrodeposition bath of claim 9 wherein the crystalline chromium deposit is substantially free of macrocracking.
 18. A process for electrodepositing a crystalline chromium deposit on a substrate, comprising providing the electrodeposition bath of claim 1; immersing a substrate in the electrodeposition bath; and applying an electrical current to deposit a crystalline chromium deposit on the substrate.
 19. A process for electrodepositing a crystalline chromium deposit on a substrate, comprising: providing an electroplating bath comprising trivalent chromium, an organic additive and at least one source of divalent sulfur, and being substantially free of hexavalent chromium; immersing a substrate in the electroplating bath; and applying an electrical current to deposit a crystalline chromium deposit on the substrate, wherein the chromium deposit is crystalline as deposited.
 20. A process for electrodepositing a crystalline chromium deposit on a substrate, comprising: providing an electroplating bath comprising trivalent chromium, an organic additive, and substantially free of hexavalent chromium; immersing a substrate in the electroplating bath; and applying an electrical current to deposit a crystalline chromium deposit on the substrate, wherein the chromium deposit is crystalline as deposited and the crystalline chromium deposit has a lattice parameter of 2.8895+/−0.0025 Å. 